Multicasting

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Multicasting is a type of Datagram Socket. Unlike regular Datagrams, Multicasting doesn’t handle each client individually instead it sends it out to one IP Address and all subscribed clients will get the message.

Example code for a server side:

public class Server {
    
    private DatagramSocket serverSocket;
    
    private String ip;
    
    private int port;
    
    public Server(String ip, int port) throws SocketException, IOException{
        this.ip = ip;
        this.port = port;
        // socket used to send
        serverSocket = new DatagramSocket();
    }
    
    public void send() throws IOException{
        // make datagram packet
        byte[] message = ("Multicasting...").getBytes();
        DatagramPacket packet = new DatagramPacket(message, message.length, 
            InetAddress.getByName(ip), port);
        // send packet
        serverSocket.send(packet);
    }
    
    public void close(){
        serverSocket.close();
    }
}

Example code for a client side:

public class Client {
    
    private MulticastSocket socket;
    
    public Client(String ip, int port) throws IOException {
        
        // important that this is a multicast socket
        socket = new MulticastSocket(port);
        
        // join by ip
        socket.joinGroup(InetAddress.getByName(ip));
    }
    
    public void printMessage() throws IOException{
        // make datagram packet to recieve
        byte[] message = new byte[256];
        DatagramPacket packet = new DatagramPacket(message, message.length);
        
        // recieve the packet
        socket.receive(packet);
        System.out.println(new String(packet.getData()));
    }
    
    public void close(){
        socket.close();
    }
}

Code for running the Server:

public static void main(String[] args) {
    try {
        final String ip = args[0];
        final int port = Integer.parseInt(args[1]);
        Server server = new Server(ip, port);
        server.send();
        server.close();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
}

Code for running a Client:

public static void main(String[] args) {
    try {
        final String ip = args[0];
        final int port = Integer.parseInt(args[1]);
        Client client = new Client(ip, port);
        client.printMessage();
        client.close();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
}

Run the Client First: The Client must subscribe to the IP before it can start receiving any packets. If you start the server and call the send() method, and then make a client (& call printMessage()). Nothing will happen because the client connected after the message was sent.

Found a mistake? Have a question or improvement idea? Let me know.

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